As is known, some applications require high hardness steel gears. For this purpose, the steel undergoes a hardening process consisting of a phase of case-hardening or nitriding of the surface and subsequent thermal treatment of the steel component. This process achieves a partial structural transformation of the steel from austenite to martensite, the grains of which make the steel harder. From a thermal point of view, the presence of grains of martensite inhibits the transport of heat due to the high thermal contact resistance at the edge of the grain.
The macroscopic result is that as the martensite content increases, the hardness of the steel increases and the conductivity and thermal diffusivity decrease. The case-hardening and/or nitriding depth is a gear design requirement and it must therefore be measured at the testing stage.
Currently, to evaluate the hardness of the samples at the end of the hardening process, the effective case-hardening or nitriding depth is measured both after the thermal treatment and downstream of the further steel machining phases, typically downstream of the final gear grinding operation.
For this purpose, after the thermal treatment, the hardness profile is determined using a durometer on the central section of a cylindrical test piece which accompanies the batch during case-hardening or nitriding; after the final grinding, the hardness profiles are determined on the tooth sides, on the root radius, on the top land and on the end face, of three teeth arranged at 120° and on the gear bearing track.
In both cases, before determination of the hardness profiles by means of the durometer, preliminary operations are necessary which entail sectioning of the gear and cylindrical test piece, enclosure in resin and polishing.
The current method for determining the hardness is disadvantageous, since it is of the destructive type and entails sectioning of the gear. Furthermore, it is costly and lengthy, due to the presence of a series of preliminary operations for preparation of the samples to be measured.
The article “Reconstruction of depth profiles of thermal conductivity of case hardened steels using a three-dimensional photothermal technique”, by Hong Qu and others, Journal of Applied Physics, vol. 104, no. 11, Jan. 1, 2008, p. 113518 describes a method for determining the effective case-hardening depth. In particular, the method described entails a non case-hardened sample and a plurality of case-hardened samples having known and different effective case-hardening depths. Furthermore, a laser source transmits electromagnetic radiation excitation, at variable frequency, to each case-hardened sample; therefore, each case-hardened sample generates an electromagnetic radiation in response. According to the method, a spectrum is furthermore determined for each case-hardened sample; this spectrum is equal to the difference between the phase of the electromagnetic response radiation generated by the case-hardened sample considered and the phase generated by the electromagnetic response radiation generated by the non case-hardened sample, this difference being a function of the frequency of the electromagnetic excitation radiation. For each spectrum a corresponding minimum and the corresponding frequency at which this minimum occurs are therefore computed; in this way, the number of minimum frequencies will correspond to the number of case-hardened samples. A calibration function is then determined, which correlates the minimum frequencies with the effective case-hardening depths of the corresponding case-hardened samples. The electromagnetic radiation excitation is then transmitted to the unknown sample, so that the unknown sample generates a respective electromagnetic response radiation. A further spectrum is subsequently determined, equal to the difference between the electromagnetic response radiation phase generated by the unknown sample and the electromagnetic response radiation phase generated by the non case-hardened sample, this difference being a function of the frequency of the electromagnetic radiation excitation. Lastly the minimum of this further spectrum is determined, and the corresponding frequency at which this minimum occurs. By comparison between the minimum frequency of the unknown sample and the above-mentioned calibration function, the effective case-hardening depth of the unknown sample is estimated.
Although the method described is of the non-destructive type, it provides results that depend largely on the precision with which the minimum frequencies of the known case-hardened samples are determined. Unfortunately, however, determination of the minimum frequencies may be imprecise and therefore the calibration function may not be accurate. Furthermore, the calibration function has a high slope; consequently, the inevitable inaccuracies in determination of the minimum frequency of the unknown sample entail considerable variations in the effective case-hardening depth determined.